A HVDC (High Voltage Direct Current) system supplies power by converting AC power from a power plant into DC power, transmitting the DC power, and then inverting the DC power into AC power at a power receiving point. The HVDC system has a loss of power less than an AC transmission type, so it has high power transmission efficiency. Further, the system can improve stability through line separation and has small inductive disturbance, so it is advantageous in long-distance power transmission.
The HVDC system is installed in a structure called a converter module composed of a plurality of submodules stacked 10 meters high in a plurality of layers. The submodules each weigh 200 kg or more, so an aerial ladder is needed to maintain the submodules, which is troublesome. It is difficult to construct an overhead traveling crane in substation facilities including a converter module, so aerial ladders are generally used.
It is very dangerous and difficult work to move submodules that are heavy equipment located in high-rise structures using an aerial ladder. Aerial ladders are usually operated by hydraulic pressure, so even if the load capacity is set larger than the weight of a submodule, a sudden load applied by heavy equipment suddenly changes the center of gravity of the aerial ladders, thus causing safety risks.
Further, considering the weight of submodules, it is very difficult and dangerous for a worker to move the submodules in a structure using an aerial ladder. In particular, it is difficult and dangerous to take out a submodule, which is heavy equipment, on an aerial ladder.